1. Field of the Invention
The present invention relates to a semiconductor acceleration sensor for automotive applications, and relates particularly to a semiconductor acceleration sensor used with air bags.
2. Description of the Prior Art
There has been known a semiconductor acceleration sensor for detecting vibration and acceleration wherein a semiconductor substrate having a thin-film diaphragm at a middle portion thereof is cantilevered at one end thereof. On the thin-film diaphragm, plural piezoresistance elements are formed to detect a force exerted thereto based on changes in the resistances of the piezoresistance elements.
FIG. 11 is a cross section of a conventional semiconductor acceleration sensor using a hermetically sealed package.
Referring to FIG. 11, semiconductor acceleration sensor 100 is a hermetically sealed package comprising a cap 101 and stem 102 with plural conductive leads 103. The cap 101 is a box-shaped member of which the side with the greatest area is open. The open edge of this cap 101 is welded to the flat panel-like stem 102 such that the stem 102 closes the box-shaped container.
A thick-film substrate 104 is bonded to the stem 102. A pedestal 106 is bonded to the thick-film substrate 104, and a sensor chip 105 is bonded to the pedestal 106 in a cantilevered manner. The thick-film substrate 104 and sensor chip 105 are connected by bonding wires or other lead wires 107. By thus using a metal package, the semiconductor acceleration sensor is shielded from operating errors induced by electromagnetic interference from such sources as engine noise, radios, and portable telephones. It is, however, difficult to reduce the size and cost of the semiconductor acceleration sensor when a metal package is used.
As a result, capacitance-type semiconductor acceleration sensors that electrically detect acceleration from the change in the capacity of the capacitor have been used as semiconductor acceleration sensors because of the ability to thereby reduce the package size and cost.
FIG. 12 is a partial cross section of a conventional capacitance-type semiconductor acceleration sensor, and FIG. 13 is a cross section along line IV--IV in FIG. 12.
As shown in FIG. 12 and FIG. 13, this capacitance-type semiconductor acceleration sensor 110 comprises a sensor chip 111 for sensing acceleration and converting acceleration to a change in electrostatic capacity; an IC chip 112 for converting the change in electrostatic capacity output by the sensor chip 111 to an electrical signal, and applying a particular signal processing operation to the resulting electrical signal; a die pad 113 to which the sensor chip 111 is die bonded; lead terminals 114 for electrical connection to the circuit board; bonding wires or other lead wires 115 for connecting the sensor chip 111 and IC chip 112, and the IC chip 112 to the external lead terminals 114; and a molded resin package 116.
Inside the sensor chip 111 are a fixed electrode and a moving electrode that is displaced by the force of inertia at acceleration. The dielectric constant between the fixed electrode and moving electrode is constant at a constant gap therebetween. As a result, the capacitance of the capacitor formed by the dielectric between the fixed electrode and moving electrode changes when the electrode distance between the fixed electrode and moving electrode changes due to displacement of the moving electrode at acceleration. It is therefore possible to detect acceleration by detecting this change in electrostatic capacity using the IC chip 112. However, the change in electrostatic capacity is extremely small, specifically on the order of 1 pF or less, and is therefore susceptible to the effects of external noise.